The present invention relates generally to separable electrical power cable connectors of the rod and bore type which are capable of loadbreak and loadmake operation and relates particularly, but not exclusively, to such connectors which have a movable piston for operating a bore contact in response to a predetermined current condition on loadmake operation.
Separable connectors are used for, among other things, interconnecting secondary electrical power circuit components, such as residential power transformers. The connectors have an insulating housing which may be provided with conductive shielding layers on the outside and inside surfaces and a conducting insert member for carrying current. The insert includes a first bore contact which will interact with a matching second rod contact of a mating second connector to complete the electrical connection.
The users of separable connectors have considered it desirable that the connectors be capable of operation while the cable is energized and feeding a load. Therefore, various features have been developed for coping with the arcing problems associated with loadmake and loadbreak operations of connectors, especially those for loadmake under fault conditions. One very significant feature which has proved to be very effective for laodmake operation under fault current conditions is the rapid movement of a bore contact to make the connection with a rod contact in response to the current through the connector. Connectors with such a feature, as well as discussions of their mode of operation are described in U.S. Pat. Nos.:
3,542,986 issued Nov. 24, 1970 to E. J. Kotski PA1 3,945,699 issued Mar. 23, 1976 to A. C. Westrom PA1 3,930,709 issued Jan. 6, 1976 to R. J. Stanger et al.
Piston operated bore contacts are usually incorporated directly in a bushing or in a switch module. The switch module is a unit which is inserted at one end into a bushing well, and which at the other end has a receiving bore and an interface surface which can be coupled with a matching elbow module having a rod contact to be pushed into the bore and engaged by the bore contact.
The switch module can be considered to have two major components. One component is an elastomeric housing with an outer conductive elastomer shield layer and an inner conductive elastomer shield layer in an elongated receiving passageway extending into the housing. The other component is a switch insert. The insert includes a container made up of a metal container tube and insulting nosepiece. Inside the container is a snuffer-contact assembly including a piston, a bore contact, and a snuffer with ablative material in the bore. The container is electrically connected to the terminal of the bushing and the piston is electrically connected to the container tube by braided wire leads.
In operation, the gas that is generated during a loadmake with excessive current, such as when there is a fault in the circuit, passes through a gas port to a retaining chamber at the inner end and drives the piston and the attached contact and snuffer instantly toward the rod contact to complete the connection operation. This permits clearing of the fault current by a circuit breaker or fuse elsewhere in the system more capable of handling such a current and prevents a possibly violent failure of the connector.
One problem with present switching inserts has been the diameter of that portion the container which houses the piston for driving the contact. A certain minimum amount of driving surface is needed on the piston face to provide the force for pushing the contact with sufficient speed. However, the attachment of the conducting braids to the piston effectively reduces the driving surface by an amount equal to the cross-sectional area of the braids. Also, the central part of the piston must be open to accommodate a gas port for flow of the gas generated in the bore to the gas retaining chamber of the container. With these constraints, a piston with sufficient driving surface becomes so large that at least the portion of the container in which the piston is disposed is larger in diameter than is that part of the container at the nose piece, where the dimensions are limited by the necessary compatibility with matching elbow modules.
The enlarged diameter of the container has some important practical consequences for manufacture of the module. In order that the insulation adhere tightly to the container for mechanical integrity, the insulation is molded around the container. The internal shield is first applied to the outer surface of the container in the form of a coating of conductive elastomeric paint. Then the container is fixed in a mold and the insulation injected at high pressures, on the order of about 4000 pounds per square inch (about 280 Kg/cm.sup.2). Because the braids must be connected between the piston and the container bottom, and because that portion of the container in which the piston is situated has a larger diameter than do other portions of the container nearer the open end, the piston cannot later be inserted from the open end. Hence, it is necessary that the piston be included in the container prior to the molding step.
One problem with the prior structure as described above is that the brazing of the braids to the piston and container bottom requires additional labor and can lead to a defective insert.
Another problem with this prior structure is that with the piston and braids inside the container in the molding step, it is not feasible to install a mandrel or core pin into the container during the molding to prevent deformation of the container wall from the high pressures. It is therefore necessary to make the container wall thick enough to withstand the pressures unassisted. This added wall thickness results in excess material costs for material which is not otherwise needed for the functioning of the device itself.
A third problem with the prior structure is that if the module should, after the molding step, prove to be defective as to the insulation only, then the container, braids, and piston must be scrapped along with the insulation, for they have become inseparable parts of the assembly. Since most of the defects in the molded assemblies are in the insulation, rather than in the container or the piston, the scrapping of good containers and pistons along with the insulation is a very significant factor in the cost of the end product.
In order to avoid the above problems, as well as to provide other manufacturing and operational benefits, there is provided in accordance with the present invention a novel module having a switch insert structure permitting a reduction in the diameter of the switch insert while maintaining the desired operational capabilities of the piston therein.